The present invention relates to handling apparatus and, more particularly, to handling apparatus for disassembly and reassembly of a turbine rotor.
The rotor of a large steam turbine is assembled by installing wheels having turbine buckets extending therefrom on a precisely machined shaft. In order to firmly connect the wheels to the shaft, it is customary to employ a shrink-fit of an axial hole in the wheel onto a mating surface on the shaft. In manufacturing, a shrink-fit is accomplished by heating each wheel to an elevated temperature, thereby expanding the wheel including the axial hole, and then fitting the wheel over the shaft. As the wheel and shaft approach the same temperature, the hole shrinks into a tight fit onto the mating surface of the shaft. The connection between the shaft and the wheel may be further improved using one or more keys. In a multi-stage rotor, the above shrink-fitting process is repeated for installation of each wheel on the shaft. The shaft and previously assembled wheels are maintained at a substantially lower temperature than the wheel being installed. The lower temperature may be room temperature, for example, or in some assembly procedures, a cooling fluid may be circulated through a cavity in the shaft to cool the shaft below room temperature.
An assembled turbine rotor such as, for example, the intermediate pressure or low pressure rotor of a large steam turbine may weigh on the order of hundreds of tons and a wheel may weigh on the order of twenty tons. Handling such massive materials and assembly of them with the precision required for successful operation as a rapidly rotating part of a steam turbine is a major problem. In a factory environment, the rotor is assembled with the shaft in the vertical position and each wheel is raised above the end of the shaft and lowered into position. The lowering and fitting process is critical since the clearances are on the order of just a few thousandths of an inch and the fitting surfaces are precisely machined and may be easily damaged. Deliberate speed is also necessary since, when heating of the wheel is stopped to begin assembly, it begins to cool and the clearance begins to disappear. At normal room temperature cooling rates, the clearance can disappear at the rate of about one thousandth per minute. A further complication is that heavy lift cranes capable of handling massive equipment move relatively slowly. The assembly of a turbine rotor becomes a race with time.
After a turbine rotor has been placed in service, it is routinely inspected by non-destructive testing techniques. It is not unknown to find flaws in turbine rotors which require the disassembly of such turbine rotors for machining or replacement. Conventionally, disassembly of turbine rotors has been done in the same vertical position as assembly. During disassembly the wheel to be removed has conventionally been heated as rapidly as possible by a ring of gas flames administered simultaneously to both sides of the wheel while injecting as little heat possible into the shaft. Several problems exist in this type of heating. Gas heating is relatively inefficient in terms of the heat entering the material of the wheel. In addition, there are many environments in which the large quantity of products of combustion produced by gas heating a wheel are undesirable. Furthermore, when the magnitude of the shrink-fit is relatively high, heat must be injected into the material at a rate which is very close to the rate which can produce metallurigical changes in the wheel and/or reach or exceed the stress limits of the material of the wheel. With gas heating it is very difficult to control the surface temperature from point to point on the wheel and certain regions may become excessively hot. For example, thin sections which provide relatively long heat conduction paths into the body material of the wheel may become overheated and damaged while thicker sections are properly heated. In addition, large quantities of a flammable gas are unwelcome in many industrial environments.
Reasons exist which make it undesirable to return some turbine rotors to the manufacturing facility for disassembly and repair. Performing the disassembly, repair and reassembly at the using site raises all of the problems described for the manufacturing facility as well as additional difficulties.
Floor space and head room in a power plant tend to be scarce commodities especially while a steam turbine is in the process of being disassembled for maintenance. Such maintenance may take place while other comparison servicing operations are being performed which consume floor space. When a power plant is built, a management economic decision very often restricts headroom above a working floor to less than that required to unstack or disassemble a turbine rotor in the vertical position. That is, a turbine rotor may be approximately 35 to 40 feet in length and a head room of approximately one and one-half times this value is required to accommodate lifting slings and clearance of the end of the shaft especially for turbine wheels which are further from the end.
In addition, upending a rotor requires the availability of one or more cranes capable of raising a load on the order of hundreds of tons to a height of approximately one and one half times the rotor length. Management economic decisions during the design of a power plant facility may limit the capacity of available cranes to less than the amount required. Even when such cranes are available, the job progresses slowly since the turbine rotor must be lifted and turned as many as twenty times during the process with each lift taking as much as eight or ten hours for rigging, lifting, turning and lowering.
From an economic standpoint, time is of the essence in returning a large steam turbine to operation since the owning facility must ordinarily replace its economic value with purchased energy. In the case of a large steam turbine driving an electric generator, the purchase of electricity from other suppliers to replace the electricity lost by non-operation during a generator outage may cost on the order of $200,000.00 per day. In the ideal, turbine rotor maintenance should be performed in the shadow of a required schedule for other maintenance in the facility so that the outage enforced by the other maintenance is not prolonged. This establishes an overall time requirement on the rebuilding activity, only part of which is the rotor unstacking and reassembly.
The insufficiency of head room and crane capacity in a using facility may make rotor unstacking and reassembly in a horizontal position an attractive option. However, significant problems are encountered in horizontal handling which have prevented this option from being exercised in large equipment. One of the problems arising during disassembly in the horizontal position results from the tendency of heat to rise. When an attempt is made to heat a wheel while its axis is horizontal, the upper half of the wheel tends to receive more heat than the lower half. Thus, the metal in the upper half may be driven close to its metallurgical and stress limits while the lower half may be insufficiently heated. Further, the weight of a wheel cannot merely be supported on the shaft while it is drawn off since this would damage precisely machined mating surfaces. Since a wheel may have to be moved two feet or more before it is freed from its own mating surface and may thereafter require moving for several more feet before it is clear of the shaft, external support of such a wheel requires a precision which is not commonly found.
Finally, once the wheel is freed from a shaft which is in the horizontal position, the wheel must be rotated or upended from its vertical position to a horizontal position in order to enable handling by conventional cranes and slings.
During reassembly in the horizontal shaft position, the problems of supporting and moving the wheel remain and, in addition, a problem of shaft bending due to differential thermal transfer from the wheel to the shaft may lock an unacceptable kink in the shaft when shrink-fitting is completed.
As previously noted, time is of the essence in rebuilding or servicing a turbine rotor. Thus, each operation which can be facilitated or simplified adds to the value of an apparatus or method for doing so.